Optical time domain measurements with high resolution and high sensitivity may be achieved using Optical Frequency Domain Reflectometry (OFDR). OFDR measurement technology enables many several important fiber-optic sensing technologies. One non-limiting example is distributed strain sensing. For example, commonly-owned, U.S. patent application Ser. No. 12/874,901, filed on Jul. 23, 2010, entitled “Optical Position and/or Shape Sensing,” incorporated herein by reference, describes how to use the intrinsic properties of optical fiber to enable very accurate shape calculation. In essence, the fiber position is determined by interpreting the back reflections of laser light scattered off the glass molecules within the fiber. This scatter is referred to herein as Rayleigh scatter. A change in optical length is detected in ones of the cores in the multi-core fiber up to a point on the multi-core fiber. A location and/or a pointing direction are/is determined at the point on the multi-core fiber based on the detected changes in optical length. This measurement of position and/or direction using a multi-core fiber can be performed quickly, with a high resolution, and to a high degree of accuracy.
In FIG. 1, a continuous Rayleigh scatter signal as might be measured by an OFDR system for a sensing fiber is depicted in the upper left graph with scatter amplitude plotted against delay time. Comparing the measured scatter pattern to a reference scatter pattern can produce a continuous measure of change in optical phase up to the end reflection of the sensing fiber, as depicted in the lower left graph. The derivative of this optical phase signal can be scaled to a measure of strain. However, if a local distortion exists due to an undesired signal bleeding into a measurement, as depicted the upper right graph, then a continuous measurement of phase cannot be produced, as depicted in the lower right graph. Hence, when a local distortion is present in the measurement, a continuous strain signal cannot be produced. Controlling or eliminating such distortions is desirable for technologies that require continuous measurements of strain along the length of the fiber, e.g., optical shape sensing.
A shape sensing system may include several, coordinated distributed strain sensing systems. Each distributed strain sensing system is connected to an independent optical core within a multi-core optical fiber. Each core is a waveguide. A non-limiting example of a multi-core optical fiber is shown in FIG. 2. The numerical dimensions shown in the Figure are just examples and are not limiting.
Because the waveguides corresponding to the independent cores within the multiple core shape sensing fiber are proximate to each other, cross-coupling between the sensing waveguides may occur. The likelihood of cross-coupling, or crosstalk, increases at locations where the optical fiber is physically modified, such as the case of an optical splice as depicted in FIG. 3. Light launched into a single core of the multi-core optical shape sensing fiber has the potential to scatter at discontinuities in fiber geometry as a result of the optical splice. There is a likelihood that light that scatters at this interface can couple into an adjacent core. This crosstalk produces signals that may be confused with or distort the intended measured signal.
Therefore, it would be desirable to provide a multiple channel interferometric system in which individual interferometric channel (an interferometric channel includes a measurement light path and a reference light path) measurements are not adversely affected by light from other channels. If possible, it would be useful to provide individual interferometric channel measurements that are essentially unresponsive to light from every other channel.